This invention relates to a device for detecting movement of muscles of a human body which is particularly but not exclusively designed for monitoring breathing patterns during sleep for use for example as a sleep apnea alarm.
Various previous sleep apnea alarm devices have been proposed for monitoring the breathing particularly of infants during sleep but also the breathing of adults who may have breathing difficulties.
There are a number of areas of difficulty in such devices and these difficulties have to date substantially prevented any effective device from being available in the marketplace.
Firstly there is the difficulty of initially detecting the movement of the muscles. Many arrangements have used devices which detect pressure changes caused by movement of the patient on a mattress but these devices have proven to be unreliable. Alternative techniques use a band or similar equipment attached around the patient particularly in the thoracic area. The extension and contraction of the band is then detected in some cases by strain guages and in other cases by movement of one element which varies a capacitive coupling. Again these devices have proven to be unreliable.
A second area of difficulty relates to the communication of the information from the detecting device to a receiving device mounted separately from the patient. In most cases a wire coupling is used but this is of course highly unsatisfactory and it can restrict movement of the patient and can be dangerous should the patient become entangled in the wire coupling.
A third area of difficulty relates to the analysis of the information from the detection device mounted on the patient. In most cases the analysis is very simplistic and can fail to distinguish between the required breathing patterns and any other type of movement.
Basically therefore it is absolutely essential in a device of this type to provide a device which is reliable in that it acts to trigger an alarm whenever breathing difficulties are encountered but at the same time the device must be able to properly distinguish from other conditions caused for example by movement of the patient away from a preferred detecting location which would cause the alarm to be actuated when no emergency condition is present. Such an alarm device which produces a number of false alarms will of course rapidly lose any credibility and will no longer be used.
Examples of devices of this type are shown in U.S. Pat. Nos. 4,146,885 (Lawson), 4,296,757 (Taylor), 4,433,693 (Hochstein) and 4,619,270 (Margolis). A further example is shown in Canadian patent 1,007,302 (Hardway).
U.S. Pat. No. 4,146,885 discloses a technique using pressure detection within a closed chamber underneath a membrane on which the patient lies. This technique has been found to be highly inaccurate and unsatisfactory for reliable detection.
U S. Pat. No. 4,296,757 discloses a simple mechanical switch attached to the patient with the improvement relating to a cushion device which protects the mechanical switch from being deactivated when the patient rolls over onto the position where the switch is located.
U.S. Pat. No. 4,433,693 discloses a device in which movement causes a variation in a capacitance to vary the tuning frequency of a tuned circuit. This variation is detected by transmission of a radio frequency electro magnetic field around the device attached to the patient and by detecting the variations in resonance. This device is apparently not commercially available and is unlikely to have the required accuracy of detection.
U.S. Pat. No. 4,619,270 discloses a stimulating device which acts to move the bed construction on which the patient lies to cause stimulation in an alarm condition.
Canadian Patent 1,007,302 discloses a pad on which the patient lies with movement of the patient causing variation in capacitance of a detection system within the pad. Again this device is unlikely to provide the necessary level of detection accuracy.